Honestly...I can kinda understand the issue here.
See, something that has to be considered here is the whole language barrier thing. Sakurai is, of course, Japanese, and ergo lives in Japan. He has several connections to people within the Japanese gaming industry, like Suda51 and Kamiya. So, when it comes to negiotating for characters for Smash, it's probably much easier for him do so with all the big gaming companies like Capcom, SEGA, Atlus, etc that are situated within his home country and also speak his language, as opposed to foreign-speaking companies that are on the other side of the globe. The western designed characters we have in Smash now (Diddy, K. Rool, Dark Samus) are all owned by Nintendo, so already there's much less of a hassle in that case. I think all of this probably factors into why we have yet to see a western composer brought on for Smash's soundtrack.
I don't even think Sakurai actually knows how to even really speak English. The only instance I recall on the top of my head was during the opening segment of the Smash 4 invitationals back at E3 2014 (where he blurted out a simple "HELLO EVERYONE!"). So yes, it's not a real hard and fast rule, and a western owned character in Smash Bros. isn't impossible, but it's kinda easy to see why western third parties are sort of at a disadvantage in comparison to Japanese ones.
I mean don't get me wrong, the language barrier is definitely a valid argument, but I couldn't tell you how many times I've seen people take a quote from Sakurai about "Smash having a lot of icons of Japanese gaming" to mean "Sakurai wants to keep Smash out of reach of western impurities" or something similar. Crash Bandicoot has always been one of my most wanted characters, and is currently my second most wanted that isn't already in, and I saw that kind of sentiment more than I'd care to admit, lol. Granted a lot of it was from some hardcore otaku posters, but still.
Honestly I'm hoping for the day any sort of western-made franchise (since Diddy, K. Rool, and Dark Samus were "safe" in this regard) gets a character in Smash just so the argument is dead, you know? Shovel Knight being an Assist is at least a step in the direction of the language barrier not mattering too much.
In general, it deals with the idea that the hardcore Smash fanbase (the ones who would be more likely to vote on ballot exit polls and post on forums) and the more casual fanbase have certain disconnects, leading to certain characters appearing much more popular on forums than among the general audience.
I think there's merit to the idea, but I think many go about it the wrong way. Namely, I don't think characters at the very top of online polls are as affected. After all, Sakurai's statement about K. Rool shows us that there was a strong correlation between online polls and the Smash ballot when it came to him, and K. Rool was absolutely crushing the ballot exit polls. That's why I don't think the Smash Bubble argument can be used against Banjo either.
I do, however, think there's a bigger case for certain characters being bigger outside the bubble, but not all of the ones people claim. For an example of one already in the game, Isabelle is a juggernaut of popularity among the general Nintendo fanbase, up there with Nintendo's biggest icons, but she wasn't nearly as popular among the more hardcore Smash fanbase. Granted, there aren't too many characters like this. Steve is probably the biggest I can think of.
Hell, come to think of it, K. Rool and Isabelle are probably the best analogues to Banjo and Steve. The former of each are massive fan favorites in spite of being out of the public eye for a while, while the latter of each group are hugely popular modern successes that have mass casual appeal, but get a more tepid reception among the hardcore fanbase.
The main characters that I think get hurt by the Bubble effect are characters like Krystal and Takamaru, who have smaller-than-top-tier but notable popularity among the hardcore fans but are borderline no names to the casual crowd.